Thermal capture cross-section logging, or thermal neutron die-away logging, has been in use for decades to monitor the saturation history of a reservoir in cased holes. Macroscopic thermal capture cross-section, or Σ, is sensitive to the presence of strong thermal absorbers. Chlorine is one such absorber, and chlorine is commonly present in formation water, while not present in any hydrocarbons. Thus, Σ-logging can often be used to obtain water saturation. It is performed by irradiating the formation with pulsed neutron beams and by monitoring the returning gamma radiation, which results from the capture of thermal neutrons by highly-absorbing isotopes present in the formation, as a function of time between pulses.
Σ-logging had been performed using several generations of pulsed neutron logging tools, beginning with several generations of TDT's (Thermal Decay Tools), GST's (Gamma Spectroscopy Tools), and RST's (Reservoir Saturation Tools). Reference can be made, for example, to U.S. Pat. Nos. 4,721,853 and 5,699,246. More recently some open-hole tools have incorporated Σ-logging capability, such as APS (Accelerator Porosity Sonde) and, for logging while drilling, “Ecoscope” (mark of Schlumberger). Ecoscope is described, for example, in Scott, Weller, el-Haliwani, Tribe, Webb, Stoller, and Galvin, “LWD Tool Saves Time And Money,” E & P July, 2006, and in U.S. Pat. No. 7,073,378. Experience with Σ in open hole has shown that the measurement can be used effectively to measure RXO and/or clay content, as clays are also rich in various thermal absorbers.
Accurate Σ-logging presents a number of technical and physics problems, which inspired various changes in the acquisition hardware over the years. The main goal is to measure the decay time-constant of γ radiation attenuation in the formation, by measuring the activity at various time intervals. The basic physics dictate that this decay be exponential in time. Because of strong contributions from the borehole fluids, casing, and the tool itself, the observed decay is far from a single exponential, and various schemes of acquisition have been developed to extract the needed information.
The early TDT tools, such as the dual-detector TDT-K, and its single detector predecessors, used three time gates (see W. B. Nelligan and S. Antkiew, Accurate Thermal-Neutron Decay Time Measurements Using Far Detector of the Dual-Spacing TDT—A Laboratory Study, SPE 6156, February 1979), where the latest gate was used to estimate and subtract activation background, and the ratio of the two earlier gates was used to estimate the decay rate. Large departure corrections needed to be applied to eliminate the borehole contamination and to correct for diffusion (that is, neutrons which diffuse out of the detection range or between different detection regions). A later version (see e.g. J. E. Hall et al, A New Thermal Neutron Decay Logging System—TDT-M, JPT, January 1982) TDT-M used a larger number of time windows (16) to decrease the sensitivity to the above effects, but with only limited success. The TDT-P which was an upgrade of the TDT-M, incorporated the dual-burst scheme, with the idea that the first, very short burst was dominated by the borehole, while the subsequent, longer burst was more affected by the formation (see e.g. D. K. Steinman, et al, Dual-Burst Thermal Decay Time Logging Principles, SPE 15437, SPE Formation Evaluation Journal, June 1988). The data were fitted to a two-exponential response model.
The results for the TDT-P were excellent where the fits were possible and of good quality, but in a number of situations (e.g. large fresh or oil-filled borehole) the fits were unstable and results not always useable.
A more recent generation of reservoir monitoring tools, the RST's incorporated a dual-burst scheme with a large number (126) of time-windows (see C. Stoller et al, Field Tests of a Slim Carbon/Oxygen Tool for Reservoir Saturation Monitoring, SPE 25375, February 1993). Instead of the dual exponential fitting, an empirical approach was used, relating moments of the time-based decay data related to a large laboratory (EECF) data base. This approach has been successful, as long as the logging situation is within the span of the EECF data base.
Application of Σ-logging in open hole presents some special problems. In a cased hole, the presence of casing, which is strongly absorbing (iron has a large thermal capture cross-section), and commonly used saline completion fluid, causes the borehole signal to decay much faster than the formation, so at later times the formation signal can be cleanly observed. By contrast, in open hole, especially in fresh muds, separation of the borehole and formation signals can be much more difficult.
The most recent entry in obtaining Open Hole Σ measurement, first referenced above, is the Ecoscope while-drilling tool, which measures Σ in addition to the more conventional evaluation measurement such as bulk density, neutron porosity and formation resistivity. Because of the inherent problem with separation the borehole and formation signals in open hole, the Ecoscope incorporates a complex pulsing scheme, using short neutron bursts to try to look at the borehole signals, and grouping sets of these with looking at later time to see the formation signal.
The U.S. Pat. No. 5,973,321 discloses a method for determining the fractional amounts of, and the thermal neutron capture cross-sections of, individual components which are included in the decay spectrum measured by a pulsed neutron well logging instrument. The method includes generating a data kernel which is made up of “representors” of decay components of the wellbore and of the earth formations in the vicinity of the instrument. The decay spectrum measured by the instrument is inverted in an attempt to determine parameters by which the representors are scaled so that, in combination, the scaled representors most closely match the measured decay spectrum. The parameters represent the fractional amounts of each exponential decay component which makes up the measured exponential decay spectrum. Gamma rays detected by the logging instrument are segregated into about 100 discrete short interval time windows called gates, with each gate corresponding to a different time interval after the end of each of a number of neutron bursts.
In the technique of the '321 patent, the time value for a gamma ray counting rate associated with each gate is stated to be preferably assigned to the midpoint of the time span for each gate (called the “center time” for each gate). Also in the technique of the '321 patent, each representor has a single value of exponential decay rate τ, and it is indicated that the number of values of τ used to generate the data kernel is preferably not greater than the number of gates in the measured data.
In some circumstances, the type of approach suggested in the referenced '321 patent may provide useful output. However, the type of approach suggested in the '321 patent has certain limitations and drawbacks. One example is the suggestion that the number of representors of exponential decay rate in the data kernel should not be greater than the number of gates in the measured data. Another example is the assignment of a single time, i.e. the midpoint, for each time window. These and other aspects of the approach of the '321 patent would limit the usefulness of the '321 technique, especially if one attempted to use the '321 technique in a complex pulsing/acquisition scheme.
It is among the objects of the present invention to provide improved techniques for accurate determination of thermal capture cross sections of formations surrounding an earth borehole.